PURPOSE: To obtain an extension circuit where constitution is comparatively simple as the extension circuit of the key telephone unit and hand-free answer back is possible, by using the speaker in the key telephone set as a microphone also.

CONSTITUTION: In a calling telephone set, a calling signal is sent from calling circuit 36 by the extension calling operation to drive called detecting circuit 25 of the called telephone set through extension control circuit 5 of the main unit, and further, called switch 26 is made to form a sound loop. Next, extension calling sound is spoken loudly from transmitter and receiver 35 by the loop passing through loss inserting circuit 6 and amplifying circuit 7. Next, for answer back to speaker 27 of the called subscriber, sound is converted to an electric signal and is sent to transmitter and receiver 35 of the calling telephone set by the loop passing through loss inserting circuit 10 and amplifying circuit 11.